KANBrief 1/22
Power take-off (PTO) drive shafts are removable components for transmitting power between a tractor and another machine. The drive shaft itself is very durable. However, its protective shrouding (guard) can wear out if the device is not used properly. If this guard is not replaced, accidents may occur which may even be fatal. A KAN expert report provides information on the scope for improvements to the standards governing PTO drive shafts.
PTO drive shafts are used in agriculture to transmit linear or rotational force from an engine unit to various machines, for example from a tractor to a machine for tillage or sowing field crops. They are also used in forestry, by local authorities, and occasionally in the construction industry. The same PTO drive shafts can be used to connect different combinations of tractor and driven machines (such as hay tedders or mowers).
The EU Machinery Directive requires “removable mechanical transmission devices” to be designed in such a way that all moving parts are safeguarded during operation and an operator is prevented from coming into contact with them. Due to their high speeds and torques, accessible rotating parts present a risk of clothing, for example, becoming wrapped around them, resulting in serious accidents. Non-rotating guards shrouding the rotating parts are intended to prevent such accidents, and constitute current good practice in the European Union. The guards for PTO drive shafts are described in standards.
Despite this measure, a fatal accident involving removable PTO drive shafts occurs on average once every two years in Germany. In Italy, the accident figures are even higher; obsolete equipment is probably a greater factor here.
Many of these accidents occur because the guard of the PTO drive shaft is damaged, has been tampered with, or is missing altogether. The conditions of use in agriculture and forestry are demanding. Soiling, weather conditions in outdoor use and frequent coupling and uncoupling place particular stress on the removable PTO drive shafts and their guards over their service life. Defective guards, whether on the transmission device itself or the driving or driven side, are often not replaced. The overhead of replacement and procurement of the parts required may be contributing factors. The device fulfils its intended purpose, i.e. transmission of the power, even when the guard is missing. Concepts familiar in industry, such as electro-sensitive protective equipment and interlocking devices, are virtually impossible to implement. Since the PTO drive shaft is employed between a towing vehicle and a machine, the guard is also constantly subjected to stress caused by movement and shocks. This is the case not only during transmission of power, but also for example when the combination of vehicle and machine is driven on the field or the road. This requires a certain flexibility of the guard. It must not be too rigid, and removal must also be possible. As yet, no solution for electronic monitoring of the guard is ready for market launch.
Common scenarios are that a chain with the function of preventing the guard from rotating with the drive shaft is not fitted securely; defective guards are not replaced; and guards are removed deliberately owing to inherent design faults. From an occupational safety and health perspective, such scenarios constitute reasonably foreseeable misuse. A requirement of the Machinery Directive however is that mechanical transmission devices must not give rise to a hazard to persons even in the event of reasonably foreseeable misuse. This must be considered at the design stage.
KAN has commissioned a detailed study into the current state of scientific and technical progress in PTO drive shafts on agricultural machinery. The report was produced by the chairs of Labour Engineering and Agricultural Systems and Technology at TU Dresden.
Analysis of existing PTO drive shafts revealed potential for improvement in standardization in some areas. In the view of the consultant experts, the fitness for purpose of PTO drive shafts could be improved by technical measures. The locking elements on the PTO drive shaft should be operable without the use of force, and lubrication in situ should be made easier. Optimization of the material used for the guard should also be discussed, in the interests of reducing wear as far as possible. Another important point is facilitating spare parts procurement, and providing the operating personnel with information on proper use and the required maintenance of the PTO drive shaft. Some potential also exists for improvements to the machines used in conjunction with PTO drive shafts . A facility on the driven machine for support of the PTO drive shaft when not in use reduces its wear.
The results of the expert report were discussed at the end of 2021 by representatives of manufacturers, standards organizations and OSH stakeholders at a KAN expert meeting. The results of this discussion are currently being collated by the KAN Secretariat, after which they are to be submitted to the standards development process. The ISO standards governing PTO drive shafts are scheduled for revision in 2022. The European standard governing PTO drive shafts was issued in 2020. The results of the KAN expert report will be submitted at the next revision process.
Katharina von Rymon Lipinski
vonrymonlipinski@kan.de